In general, an automobile body has metal members that form the body and plastic members that form bumpers and the like.
When coating such automobile bodies in the past, it was commonly adopted for the metal members and plastic members to be coated using different paints and different coating processes that were suitable for the respective members, after which the plastic members were fitted to the metal members.
In recent years, however, there has been a demand for methods for carrying out coating in a state whereby plastic members are already fitted to metal members in order to lower equipment costs in automobile body production processes and harmonize color tones between metal members and plastic members.
For example, PTL 1 discloses a method for coating an automobile outer panel part, which is characterized by assembling metal members, which have been electrodeposited in advance with a cation type electrodeposition paint, and plastic members so as to obtain an automobile outer panel part, coating both types of member with an aqueous barrier coat containing a composition comprising an olefin-based resin and a urethane-based resin as a primary automobile component, then applying an intermediate paint if necessary, and then applying a top paint. In addition, PTL 2 discloses a method for forming a coating film, which is characterized by integrally assembling a metal member, which has been subjected to electrodeposition painting, and a plastic member, coating the surfaces of both members with a solvent-based paint which contains an isocyanate-modified polybutadiene resin, a polyol component having a number average molecular weight of 150 to 50,000 and an electrically conductive filler as primary components and which can form a coating film having a static glass transition temperature of −100° C. to 0° C., then applying an intermediate paint if necessary, and then applying a top paint. However, these coating methods cannot be said to be satisfactory in terms of impact resistance of an obtained multilayer coating film in a low temperature environment (−30° C.).
With regard to ensuring physical properties in low temperature environments, PTL 3 discloses a method for forming a multilayer coating film in which an electrodeposition coating film, an intermediate coating film, a base coating film and a clear coating film are sequentially laminated on a substrate and in which the clear coating film has a tensile strength of 60 MPa or more and a breaking elongation rate of 5% or more at −20° C., but this method is not suitable for substrates having both metal members and plastic members and is insufficient in terms of impact resistance in a −30° C. environment.